Instrument Information
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| IDENTIFIER |
urn:nasa:pds:context:instrument:nsp1.lcross::1.1
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| NAME |
NEAR INFRARED SPECTROMETER 1
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| TYPE |
SPECTROMETER
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| DESCRIPTION |
Instrument Overview =================== The LCROSS payload contains two near-infrared spectrometers (NSP1/NSP2). Their electronics units are identical, but they have different fore-optics designs. Near infrared spectrometer #1 (NSP1), also known as the nadir NSP, is located inside the Payload Observation Deck and is orientated along the +X axis in LCROSS spacecraft coordinate system. The payload bore-sight is defined as the central position of the NSP1 bore-sight throughout the mission. During the final hour of the mission, the LCROSS spacecraft's +X axis was orientated towards the lunar impact site. The payload's boresight (center of the NSP1 one-degree field of view (FOV)) is targeted towards the expected Centaur impact location. In comparison, the fore-optics feeding near infrared spectrometer #2 (NSP2), also known as the solar-viewer or occulting NSP, is orientated approximately along the -Z axis in LCROSS spacecraft coordinate system. This fore-optic is expected to collect radiance from the sun in its generous approximate 130 degree FOV during the final hour of the mission. Both spectrometers provide 1.2-2.4 micron spectral coverage at low resolution. Both spectrometers are manufactured by Polychromix, a company whose primary spectrometer line is designed for material analysis and chemical sensing. The spectrometers are designed to take a single SMA NA=0.22 fiber as input instead of a slit. The diameter of this fiber defines the resolution of the instrument. Within the electronics unit of the spectrometer, light collected from the fiber (plus fore-optics) is reflected off a grating to spread frequencies across an innovative, electronically-tunable MEMS device. This MEMS device reflects selected frequencies onto a single, TEC-cooled InGaAs sensor element. The combination of frequencies merged together changes according to a timed sequence, and the resulting measurement stream is decoded on the ground to recover the individual frequencies. The result is a highly sensitive, low power, and inexpensive spectrometer. The spectrometer contains no moving parts other than the MEMs device and the peak power for each NSP is 2.5 W. NSP1 is fed by 75 cm length 600-micron core-diameter low-OH glass fiber attached to a fore-optic in the Payload Observation Deck (POD). The fore-optics unit is a fixed two-mirror and one lens system designed to provide one degree circular field-of-view. Scientific Objectives ===================== The two LCROSS near infrared spectrometers (NSP1/NSP2), by providing R~40-70 spectra over the 1.2 to 2.4 micron wavelength region, are the primary method to measure the amount of water vapor in the ejecta. The LCROSS spacecraft will monitor spectral bands associated with water vapor, ice, and hydrated minerals covering the first overtones of the symmetric and asymmetric stretches of water. This band, relatively free from interferences, is more brightly illuminated by sunlight than the fundamentals near 3 microns, more than compensating the weaker absorption of the overtones. The regions near 1.4 and 1.9 microns, normally obscured by terrestrial atmospheric background in spectra from icy surfaces, will provide a sensitive indication of water vapor from ice or hydrates. The remainder of the spectral band will reveal the nature of ice crystals and mineral hydrates. In addition, the nadir NSP1, can be operated in a fast spectral mode to provide a coarser spectral sampling, but at faster sampling rate (75 Hz versus 1.7 Hz normal spectral mode), to capture the spectra shape and radiance of the infrared impact flash event. Calibration =========== Spectra generated by the near infrared spectrometers are initially processed by the LCROSS GSEOS (Ground Support Equipment Operating System) which extracted the CCSDS files from telemetry, applied a fixed mathematical function (the Hadamard transform provided by the manufacturer) to convert 256 mask positions to 100 spectral elements, and converted them to an ASCII comma-separated file with some metadata and with MET-based packet timestamp encoded in the filename. Spectra taken in Flash or Diagnostic Mode are saved into one file appended by subsequent sampling. Wavelength calibration, mapping pixel to wavelengths in microns, is provided by cross-checking in-orbit data against lab reference spectra taken on the ground. Wavelength calibration is applied only to the 100 pixels for Hadamard Mode, and a look-up table providing a the wavelength range across each mask position in Flash Mode. Conversion from raw data values [DN] to a scene spectral radiance [W/m^2 um sr], with errors, is performed using a response curve measured by the instrument pre-flight using a NIST-calibrated reference source. As the pre-flight calibration data was performed at atmosphere, particular attention has been paid to addressing and correcting for any water contamination in the calibration products. The calibration steps are described in the LCROSS Instrument Response and Calibration Report in the CALIB directory of this archive. For Hadamard spectra, a separate additional raw (pre-Hadamard transform) ASCII file containing the original mask positions, will also be part of the PDS delivery. Operational Modes ================= Both spectrometers have three modes: Impact Flash (IF), Hadamard Spectrum (HS) and Diagnostic (DI), which are configurable by sending a command to the unit. The nominal Hadamard Spectrum Mode provides a 100-pixel spectra at 1.7 Hz rate with continuous coverage across 1.2 to 2.4 microns. The Impact Flash mode provides a five point (with a dark mask) sampling at known wavelengths across the spectral range at a 72 Hz sampling rate. The mask wavelengths, set approximately around the water bands, are described in the Instrument Response and Calibration Report. The instrument mode will be documented with each spectra or mask product.
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| MODEL IDENTIFIER |
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| NAIF INSTRUMENT IDENTIFIER |
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| SERIAL NUMBER |
not applicable
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| REFERENCES |
unk
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